Philips TV: Irregular automatic shutdown
Fault Phenomenon: Irregular automatic shutdown accompanied by a buzzing sound. After turning off the power and waiting for several minutes before restarting, the same fault phenomenon repeats.
Circuit Features Analysis:
(1) This model uses the Hitachi NP82C core circuit;
(2) The switch power supply circuit is composed of the STR6020 thick-film circuit and peripheral components, belonging to a self-excited line-synchronous series single-power voltage (111V) output circuit;
(3) There is an overvoltage and overcurrent protection circuit, which is formed by a thyristor Q751 along with CP751 and ZD751. The sampling voltage for the overvoltage protection circuit is taken from the pulse at pin ③ of the line output transformer T771, then rectified through R771, C775, and D751 to obtain a stable voltage via CP751. The sampling voltage for the overcurrent protection circuit is taken from the voltage drop across resistor R783 on the emitter of transistor Q751, and stabilized by Zener diode ZD751.
Maintenance Techniques:
(1) Determine whether the protection circuit has been activated by monitoring the control gate voltage of thyristor Q751. When a fault occurs, the voltage drops to 0.7V, indicating that the protection circuit has been triggered (the buzzing sound confirms this);
(2) To determine whether it's an overvoltage, overcurrent, or internal fault in the protection circuit itself, a simple method is to disconnect the protection circuit (by removing R751) and use a dummy load (a 60W bulb) to test the unit. If the 111V voltage rises above 140V, it indicates that the direct cause of the fault is a mis-triggering of the overvoltage protection circuit;
(3) By testing and analyzing, determine the cause of the overvoltage, which may be within the switch power supply sampling and stabilizing control circuit. Upon checking the sampling circuit including WOI12 winding, m07, and 0907, all were found normal. However, there was suspicion of issues with the error amplifier and pulse width modulation circuits inside the STR6020 integrated circuit. After replacement, the fault was resolved.
Cause Analysis: The fault was caused by degradation of the internal circuitry (stabilizing control loop) of the STR6020 integrated block, leading to an increase in the power supply output voltage. The pulse output at pin ③ of the line output transformer T771, after being rectified through R771, C775, and D751, resulted in an increased voltage, thereby triggering the conduction of thyristor Q751, causing the overvoltage protection circuit to malfunction.
Source: Beijing Philips TV Repair Center http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3761179.